The present invention relates to a multimold semiconductor device and the manufacturing method therefor.
Recently, multimold semiconductor devices molded by two or more resin layers of different characteristics have been widely used. One of the most typical multimold semiconductor devices is a double mold photocoupler.
The multimold photocoupler devices as described above have been so far manufactured in accordance with the following process: first, semiconductor chips including light-emitting elements and light-receiving elements are mounted on a lead frame and then wires are bonded thereon. Thereafter, the semiconductor chips are arranged in cavities of an inner resin mold and an inner resin is injected into the inner resin mold. The chips sealed by the hardened inner resin are removed from the inner resin mold. In this process, wax serving as a mold releasing agent or mold lubricant is added to the inner resin prepared in the form of tablet, so that the hardened inner resin sealed chip can be easily removed from the mold. The principal component of the conventional wax added to the inner resin is hydrocarbon. The characteristics of the conventional wax are as follows: (1) the external activity is high and the exudation rate to resin surface is high; (2) the molding point is relatively high beyond 100.degree. C.; (3) the viscosity at higher temperature is high; (4) the polarity is small and the compatibility with another wax composed of other components is inferior; (5) the oxidization rate in air is high and therefore the brittleness due to oxidation is large. Therefore, when wax including hydrocarbon as the principal component is added to the inner resin, there exists a problem in that a high viscosity wax layer is easily formed on the outer surface of the inner resin. Since an outer resin is formed on the inner resin so as to enclose the inner resin, when wax remains on the outer surface of the inner resin, the wax on the inner resin is difficult to flow and move during the outer resin forming process and further, the conformability with the wax added to the outer resin is low, thus raising various problems in that the adhesion strength at the boundary between the inner and outer resins is low and therefore the peeling strength is also low. Additionally the moisture resistance and the heat releasability are deteriorated.
To overcome these problems, inner resin treating process has been so far indispensable. The inner resin treating process includes a honing process for polishing the inner resin surface, a burning process for removing the wax exuded on the inner resin surface by burning, after-curing process for stabilizing the bridge formation reaction of the inner resin, etc. The semiconductor chips sealed by the inner resin thus treated are arranged in cavities formed in an outer resin mold; an outer resin is injected thereinto; and the chips enclosed by hardened outer resin are removed from the mold. However, the inner resin treating process results in an increase in the cost and a decrease in the yield.
In connection with this, in the case of simple-shaped double mold devices, since a single and simple inner resin treating process may be sufficient, there exists no serious problem. However, in the case of multimold (three or more mold) devices, the inner resin treatment process is rather complicated and therefore costly. Therefore, there exists a strong request to eliminate the inner resin treating process. However, it has been extremely difficult to prevent the boundary between the inner and outer resins from being peeled or separated without inner resin treatment.
Furthermore, when the inner resin treatment is not effected, there exists another problem in that the moisture resistance is low in addition to low peeling resistance. This is because since the adhesion strength between the inner and outer resins is low, moisture in air easily permeates into inside the chip inside through between the lead frame and the outer resin and then between the lead frame and the inner resin. Further, the low adhesion strength between the inner and outer resins causes heat generated by the semiconductor chip not to effectively conduct from the inner resin to the outer resin, so that the heat is not well radiated from the chip to air. The above-mentioned various problems result from the low adhesion strength between the inner and outer resins, and therefore it has been so far difficult to improve the adhesion strength between the two resins, without inner resin treatment.